I have been looking at the new processors and i really like the AMD phenom x2 II 550 3.0GHZ as it is cheap and good.But i have a motherboard which is pretty old but can support this new processor. i NEED you advice on if i should but the new processor and stay with my AM2+/AM3 DDR 2 motherboard or get a whole new motherboard and get this processor.

As long as your mobo supports the 550, then go for it!
As B-Shot mentioned, you will probably need to update your bios prior to installation.
I'm also considering the 550 to replace an aging Athlon x2 6000+ on a spare comp.